Complexes Containing cis-[MoVO2]+ and cis-[MoVO (OH)] 2+ Centers

2912

J. Am. Chem. Soc. 1996, 118, 2912-2921

Complexes Containing cis-[MoVO2]+ and cis-[MoVO(OH)]2+ Centers Zhiguang Xiao,* Robert W. Gable, Anthony G. Wedd, and Charles G. Young* Contribution from the School of Chemistry, UniVersity of Melbourne, ParkVille, Victoria 3052, Australia ReceiVed July 27, 1995X

Abstract: Cobaltocene reduction of LMoVIO2(SPh) complexes yielded extremely dioxygen-sensitive cobaltocenium salts of the cis-dioxo-Mo(V) radical anions cis-[LMoVO2(SPh)]-, while sodium acenaphthalenide reduction of LaMoVIO2(SPh) yielded a complex sodium salt containing [LaMoVO2(SPh)]- [L ) hydrotris(3,5-dimethylpyrazol-1yl)borate (La), hydrotris(3-isopropylpyrazol-1-yl)borate (Lb), or hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (Lc)]. Crystals of [CoCp2][LcMoVO2(SPh)]‚toluene were orthorhombic, space group Pbca, with a ) 17.695(3) Å, b ) 19.490(3) Å, c ) 21.925(4) Å, V ) 7561(2) Å3 for Z ) 8. The structure, refined using a full-matrix least-squares procedure and 2269 data, converged with R ) 0.067 (Rw ) 0.067). In the distorted octahedral anion, the metrical parameters of the cis-MoVO2 fragment [Mo-O ) 1.742(9) Å, O-Mo-O ) 112.1(4)°] are significantly larger than those of LcMoVIO2(SPh) [average Mo-O ) 1.700(6) Å, O-Mo-O ) 103.9(2)°]. 18O-Substitution of [CoCp2][La,bMoVO2(SPh)] permitted the assignment of bands at ca. 870 and 770 cm-1 to the νs and νas modes, respectively, of the cis-MoVO2 fragment. Freshly prepared solutions of [CoCp2][LMoVO2(SPh)] exhibited a broad EPR signal (g, 1.92; W1/2, 20 G) characteristic of a cis-[MoVO2]+ complex. The signal was stable when L ) Lc but when L ) La or Lb it was replaced by a proton-coupled signal characteristic of a cis-[MoVO(OH)]2+ center. The complex LaMoVO(OH)(SPh) was isolated as a coprecipitate with its conjugate base salt [CoCp2][LaMoVO2(SPh)]. It was characterized by infrared bands at 915 and 535 cm-1, assigned to ν(ModO) and ν(MosOH) modes, respectively. The anions were readily converted to air-stable LMoVO(OSiMe3)(SPh) upon reaction with Me3SiCl and reacted with dioxygen to regenerate LMoVIO2(SPh). The paper reports the first isolation in substance of compounds containing novel cis-[MoVO2]+ and cis-[MoVO(OH)]2+ centers and the first crystallographic characterization of a cis-dioxoMo(V) species.

Introduction In dioxo complexes of d0 transition metals, favorable pπ-dπ orbital interactions between the π-base oxo ligands and π-acid metal centers are optimized when a cis geometry is adopted; accordingly, all such complexes exhibit cis geometries.1,2 However, upon one- or two-electron reduction, cis-[MO2]x+ complexes appear to be thermodynamically unstable with respect to isomerization to their trans forms, which can accommodate the extra electrons in nonbonding orbitals, or to decomposition.1,2 To our knowledge, no detailed account of the isolation of any d1 cis-[MO2]x+ coordination complex has appeared. Isolable d2 cis-[MO2]x+ complexes are restricted to the elements rhenium,3-5 ruthenium,6-8 and osmium.9-12 Many of the isolated Ru and Os complexes, as well as a number of in situ generated d2 cis-dioxo-Ru and -Os complexes, are implicated X Abstract published in AdVance ACS Abstracts, February 1, 1996. (1) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds, Wiley: New York, 1988. (2) Theoretical treatments of bonding in MOx fragments include the following: (a) Griffith, W. P. Coord. Chem. ReV. 1970, 5, 459. (b) Mingos, D. M. P. J. Organomet. Chem. 1979, 179, C29. (c) Tatsumi, K.; Hoffmann, R. Inorg. Chem. 1980, 19, 2656. (d) Rappe, A. K.; Goddard, W. A., III. J. Am. Chem. Soc. 1982, 104, 448. (e) Rappe, A. K.; Goddard, W. A., III. J. Am. Chem. Soc. 1982, 104, 3287. (f) Brower, D. C.; Templeton, J. L.; Mingos, D. M. P. J. Am. Chem. Soc. 1987, 109, 5203. (g) Cundari, T. R.; Drago, R. S. Inorg. Chem. 1990, 29, 2303. (h) Peng, G.; Nichols, J.; McCullough, E. A., Jr.; Spence, J. T. Inorg. Chem. 1994, 33, 2857. (3) cis-ReO2I(PPh3)2: (a) Freni, M.; Giusto, D.; Romiti, P.; Minghetti, G. Gazz. Chim. Ital. 1969, 99, 286. (b) Ciani, G. F.; D’Alfonso, G.; Romiti, P. F.; Sironi, A.; Freni, M. Inorg. Chim. Acta 1983, 72, 29. (c) Sullivan, B. P.; Brewer, J. C.; Gray, H. B. Inorg. Synth. 1992, 29, 146. The O-Re-O bond angle of 138.7(6)° in this five-coordinate complex is nearly midway between O-M-O angles typical of cis (ca. 105°) and trans (180°) complexes.

0002-7863/96/1518-2912$12.00/0

in novel stoichiometric and catalytic oxygen atom transfer systems.13 The cis-[MoVIO2]2+ center dominates the chemistry of d0 Mo(VI) complexes14 and participates in many oxygen atom transfer reactions.15-17 With few exceptions, one-electron reduction (4) cis-[ReO2(bpy)(py)2]+: (a) Ram, M. S.; Johnson, C. S.; Blackburn, R. L.; Hupp, J. T. Inorg. Chem. 1990, 29, 238. (b) Blackburn, R. L.; Jones, L. M.; Ram, M. S.; Sabat, M.; Hupp, J. T. Inorg. Chem. 1990, 29, 1791. (c) Ram, R. S.; Hupp, J. T. Inorg. Chem. 1991, 30, 130. (5) Abbreviations used: Ac ) acenaphthalene or its radical anion; bpy ) 2,2′-bipyridine; Cp ) η5-cyclopentadienyl; L ) hydrotris(3,5-dimethylpyrazol-1-yl)borate (La), hydrotris(3-isopropylpyrazol-1-yl)borate (Lb), or hydrotris(3,5-dimethyl-1,2,4-triazol-1-yl)borate (Lc); L-N2S2 ) dianion of N,N′-dimethyl-N,N′-bis(2-mercaptophenyl)-1,2-diaminoethane; L-NS2 ) dianion of 2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine; L-N2O2 ) dianion of N,N′-bis(2-hydroxyphenyl)-N,N′-dimethyl-1,2-diaminoethane; Me3tcn ) N,N′,N′′-trimethyl-1,4,7-triazacyclononane; py ) pyridine; THF ) tetrahydrofuran; EHMO ) extended Hu¨ckel molecular orbital; INDO ) intermediate-neglect-of-differential-overlap; HOMO ) highest occupied molecular orbital; LUMO ) lowest unoccupied molecular orbital. (6) cis-[RuO2Cl3]-: Perrier, S.; Kochi, J. K. Inorg. Chem. 1988, 27, 4165. (7) cis-[RuO2(O2CMe)Cl2]-: (a) Griffith, W. P.; Jolliffe, J. M.; Ley, S. V.; Williams, D. J. J. Chem. Soc., Chem. Commun. 1990, 1219. (b) Griffith, W. P.; Jolliffe, J. M. J. Chem. Soc., Dalton Trans. 1992, 3483. (8) cis-[RuO2(Me3tcn)(O2CCF3)]+: Cheng, W.-C.; Yu, W.-Y.; Cheung, K.-K.; Che, C.-M. J. Chem. Soc., Chem. Commun. 1994, 1063. (9) cis-[OsO2(O2CMe)3]-: (a) Criegee, R.; Marchand, B.; Wannovius, H. Liebigs Ann. Chem. 1942, 550, 99. (b) Behling, T.; Capparelli, M. V.; Skapski, A. C.; Wilkinson, G. Polyhedron 1982, 1, 840. (10) cis-[OsO2(O2CMe)Cl2]-: (a) Griffith, W. P.; Jolliffe, J. M. J. Chem. Soc., Dalton Trans. 1992, 3483. (11) OsO2(diolate)(N-donor): Pearlstein, R. M.; Blackburn, B. K.; Davis, W. M.; Sharpless, K. B. Angew. Chem., Int. Ed. Engl. 1990, 29, 639. (12) cis-[OsO2(bpy)2]2+: (a) Dobson, J. C.; Takeuchi, K. J.; Pipes, D. W.; Geselowitz, D. A.; Meyer, T. J. Inorg. Chem. 1986, 25, 2357. (b) Dobson, J. C.; Meyer, T. J. Inorg. Chem. 1989, 28, 2013. (13) Goldstein, A. S.; Beer, R. H.; Drago, R. S. J. Am. Chem. Soc. 1994, 116, 2424 and references therein.

© 1996 American Chemical Society

Complexes Containing MoV Centers leads to [MoVO]3+ or [MoV2O3]4+ species, rather than stable cis-[MoVO2]+ complexes. Elimination and/or condensation reactions, initiated by coupled electron-proton transfer reactions involving an oxo ligand in the reduced complex, account for the formation of these thermodynamically favored species.15,16 Kinetically controlled syntheses, which exploit ligands capable of retarding elimination and the close approach of molybdenum centers, are required for the generation of long-lived cis[MoVO2]+ complexes. Only a small number of cis-[MoVIO2]2+ complexes are known to undergo a reversible, one-electron reduction on the cyclic voltammetric time scale.18-23 The reduction products, which can be chemically generated in many cases, are formulated as cis-[MoVO2]+ complexes on the basis of EPR studies. Some of these complexes protonate to form cis-[MoVO(OH)]2+ species, but the coligands are effective in retarding further reactions.20-25 These formulations are consistent with extensive multifrequency EPR studies of [(L-N2S2)MoO2]- and (L-N2S2)MoO(OH) complexes and their 95,97Mo-, 98Mo-, 1H-, 2H-, and 17O-labeled analogues.5,20,26,27 Possible geometries and electronic structures for these species were predicted recently using ab initio and INDO molecular orbital methods, and in most cases, the results were concordant with those obtained from solution EPR studies.2h None of these complexes have been isolated in substance, and no structural information is available. Recently, we reported a wide range of LMoVIO2X complexes, some of which exhibited a reversible, one-electron reduction on cyclic voltammetric time scales, producing cis-[LMoVO2X](X ) ER, E ) O, S; R ) alkyl, phenyl) species characterized by highly anisotropic frozen glass EPR spectra with low 〈g〉 values.5,23 Protonation led to cis-[MoVO(OH)]2+ complexes characterized by doublet EPR spectra arising from proton superhyperfine coupling.23 We report here the isolation of various salts of the cis-dioxo-Mo(V) radical anions [LMoVO2(SPh)]-, along with details of their spectroscopic, crystallographic, and electrochemical characterization. Aspects of this work have been communicated.28,29 We also report (14) Stiefel, E. I. In ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon: Oxford, 1987; Chapter 36.5, p 1375. (15) Young, C. G.; Wedd, A. G. In Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; Wiley: New York, 1994; p 2330 and references therein. (16) Enemark, J. H.; Young, C. G. AdV. Inorg. Chem. 1993, 40, 1 and references therein. (17) (a) Sheldon, R. A.; Kochi, J. K. Metal-Catalysed Oxidations of Organic Compounds; Academic: New York, 1981. (b) Holm, R. H. Chem. ReV. 1987, 87, 1401. (18) Pickett, C.; Kumar, S.; Vella, P. A.; Zubieta, J. Inorg. Chem. 1982, 21, 908. (19) Kaul, B. B.; Enemark, J. H.; Merbs, S. L.; Spence, J. T. J. Am. Chem. Soc. 1985, 107, 2885. (20) Dowerah, D.; Spence, J. T.; Singh, R.; Wedd, A. G.; Wilson, G. L.; Farchione, F.; Enemark, J. H.; Kristofzski, J.; Bruck, M. J. Am. Chem. Soc. 1987, 109, 5655. (21) Farchione, F.; Hanson, G. R.; Rodrigues, C. G.; Bailey, T. D.; Bagchi, R. N.; Bond, A. M.; Pilbrow, J. R.; Wedd, A. G. J. Am. Chem. Soc. 1986, 108, 831. (22) Roberts, S. A.; Young, C. G.; Kipke, C. A.; Cleland, W. E., Jr.; Yamanouchi, K.; Carducci, M. D.; Enemark, J. H. Inorg. Chem. 1990, 29, 3650. (23) Xiao, Z.; Bruck, M. A.; Doyle, C.; Enemark, J. H.; Grittini, C.; Gable, R. W.; Wedd, A. G.; Young, C. G. Inorg. Chem. 1995, 34, 5950. (24) Subramanian, P.; Kaul, B.; Spence, J. T. J. Mol. Catal. 1984, 23, 163. (25) Hinshaw C. J.; Spence, J. T. Inorg. Chim. Acta 1986, 125, L17. (26) Wilson, G. L.; Greenwood, R. J.; Pilbrow, J. R.; Spence, J. T.; Wedd, A. G. J. Am. Chem. Soc. 1991, 113, 6803. (27) Greenwood, R. J.; Wilson, G. L.; Pilbrow, J. R.; Wedd, A. G. J. Am. Chem. Soc. 1993, 115, 5385. (28) Xiao, Z.; Young, C. G.; Enemark, J. H.; Wedd, A. G. J. Am. Chem. Soc. 1992, 114, 9194. (29) Xiao, Z.; Gable, R. W.; Wedd, A. G.; Young, C. G. J. Chem. Soc., Chem. Commun. 1994, 1295.

J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2913 evidence for the isolation of the conjugate acid of [LaMoVO2(SPh)]-, i.e., LaMoVO(OH)(SPh). cis-[MoVO(OH)]2+ centers have been implicated in the turnover of the enzymes sulfite oxidase and nitrate reductase;15,16,30-33 this work represents the first isolation in substance of such a species. Kinetically controlled syntheses and the judicious choice of coligands for the cis-[MoVO2]+ center were crucial factors in the isolation and characterization of these extremely dioxygen-sensitive species. Experimental Section Materials and Methods. Complexes LMoVIO2(SPh) were prepared as described previously.22,23 Cobaltocene (CoCp2) was prepared by literature methods34 and purified by careful and repeated sublimation. Chlorotrimethylsilane was dried and distilled from AlCl3/CaH2. Acenaphthalene (Aldrich) was purified by chromatography on silica gel with dichloromethane as solvent, recrystallized from dichloromethane/ methanol, and dried at 40-50 °C under vacuum. Flame-dried glassware, dinitrogen atmospheres, and carefully dried and deoxygenated solvents35 were employed in all syntheses and manipulations in solution. The solvents employed for 18O isotope studies were rigorously dried anaerobically by stirring over activated alumina immediately prior to use. The 95 atom % H218O was purchased from Aldrich Chemicals. Infrared spectra were recorded on a Bio-Rad FTS-60A Fourier Transform IR spectrophotometer calibrated with polystyrene; complexes were sampled via dispersion in pressed KBr discs or via Nujol mulls between KBr plates. Air-sensitive samples were prepared in a glovebox and maintained under an inert atmosphere until the spectrum was recorded. EPR spectra were obtained on a Bruker ECS 106 EPR spectrometer using 1,1-diphenyl-2-picrylhydrazyl as reference; frozen solutions of [CoCp2][LMoVO2(SPh)] were prepared by dissolving the complex in an EPR tube under anaerobic conditions and immediately freezing the solution. Samples were melted and warmed to room temperature just before recording the liquid solution EPR spectrum. Near-IR and UV-visible spectra were recorded on Cary 17 and Hitachi 150-20 instruments, respectively. Electrochemical experiments were performed as described previously.23 Acetonitrile solutions of the compounds (1-2 mM) in 0.2 M Bun4NBF4 were routinely employed. Potentials are quoted relative to the saturated calomel electrode (SCE). Electron impact (70 eV) mass spectrometric measurements were performed on a VG Micromass 7070 F mass spectrometer. Microanalyses were performed by the Analytische Laboratorien, Engelskirchen, Germany, or Atlantic Microlabs, Norcross, GA. EHMO calculations were performed on a CAChe Tektronics system. The crystallographic coordinates of LaMoVIO2(SPh)23 and [LcMoVO2(SPh)]- were used to build molecular structures for [LaMoO2(SPh)]0/- and [LcMoO2(SPh)]0/- complexes, with O-Mo-O angles of 102.6° and 112.1°, respectively. These complexes approximate Cs symmetry which is assumed in the orbital and EPR treatments presented in this paper. EHMO calculations on [LcMoO2(SPh)]0/- based on the coordinates of LcMoO2(SPh)23 are not valid within these treatments due to the reduced (C1) symmetry of this species in the solid state.36 The presence of different L ligands had only a small effect on the outcome of the calculations, with the energies of the orbitals in the complexes of the electron-withdrawing Lc ligand being lower than those of the La complexes. (30) Molybdenum Enzymes, Cofactors and Model Systems, Stiefel, E. I., Coucouvanis, D., Newton, W. E., Eds.; ACS Symposium Series 535; American Chemical Society: Washington, DC, 1993. (31) Young, C. G.; Wedd, A. G., in ref 30, p 70. (32) Bray, R. C. AdV. Enz. Rel. Areas Mol. Biol. 1980, 51, 107. (33) Bray, R. C. Q. ReV. Biophys. 1988, 21, 299. (34) (a) Wilkinson, G.; Cotton, F. A.; Birmingham, J. M. J. Inorg. Nucl. Chem. 1956, 2, 95. (b) King, R. B. Organometallic Syntheses; Academic: New York, 1965; Vol. 1, p 70. (35) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, 1980. (36) The C1 symmetry of LcMoVIO2(SPh) is a consequence of the significant twisting of the SPh- ligand out of the mirror plane. Interestingly, the orientation of the potentially π-donor SPh- ligand impacts on the symmetry and energy of the π-manifold of the MoO2 fragment. This effect was, however, not examined in detail.

2914 J. Am. Chem. Soc., Vol. 118, No. 12, 1996 Syntheses. [CoCp2][LaMoVO2(SPh)]. A solution of CoCp2 (0.14 g, 0.74 mmol) in acetonitrile (10 mL) was filtered onto LaMoVIO2(SPh) (0.20 g, 0.37 mmol), and the mixture was stirred for 15 min. The resultant green microcrystals were filtered, washed three times with acetonitrile, and then dried under vacuum. Yield 0.19 g (70%). Anal. Calcd for C31H37BCoMoN6O2S: C, 51.47; H, 5.16; N, 11.62; S, 4.43. Found: C, 51.23; H, 5.28; N, 11.90; S, 4.33. Electrochemistry (CH3CN): E1/2(MoVI/MoV) ) -0.76 V. 18O-Enriched [CoCp2][LaMoVO2(SPh)] was prepared similarly using enriched LaMoVIO2(SPh) prepared as described below. The level of 18O in [CoCp2][LaMoVO2(SPh)] was found to be the same as that in the starting complex LaMoVIO2(SPh) by mass spectrometric analysis of LaMoVO(OSiMe3)(SPh) produced from the salt upon reaction with Me3SiCl (vide infra). LaMoVIO2(SPh) (80 Atom % 18O). A mixture of LaMoVIO2(SPh) (0.40 g, 0.75 mmol) and PPh3 (0.090 g, 0.34 mmol) was treated with alumina-dried tetrahydrofuran (10 mL) and H218O (250 mg, 12.5 mmol). The dark brown mixture was stirred at room temperature under dinitrogen for 7 h during which the solution slowly turned dark green. Dioxygen was then bubbled through the solution to regenerate the initial brown color. The solution was immediately evaporated to dryness under vacuum and chromatographed on silica gel in air using toluene as eluent. The dark brown fraction was collected, and the complex was recrystallized from dichloromethane/methanol. Yield 0.29 g (72%). The isotope-independent properties of this substance were identical to those of an authentic sample of LaMoVIO2(SPh).22,23 Infrared data were consistent with the presence of 16O16O, 16O18O, and 18O18O isotopomers. Mass spectra of the LaMoVO(OSiMe3)(SPh) derivative confirmed enrichment in 18O to the 80 atom % level: for [LaMoO(OSiMe3)]+ m/z (%) 494 (4), 495 (7), 496 (22), 497 (23), 498 (61), 499 (50), 500 (74), 501 (87), 502 (100), 503 (69), 504 (85), 505 (29), 506 (30), 507 (8). [CoCp2][LbMoVO2(SPh)]. A solution of CoCp2 (0.14 g, 0.74 mmol) in toluene (10 mL) was filtered onto a stirred solution of LbMoVIO2(SPh) (0.20 g, 0.34 mmol) in toluene (10 mL). Green microcrystals were filtered off, washed three times with toluene, and dried in vacuum. Yield 0.18 g (69%). Anal. Calcd for C34H43BCoMoN6O2S: C, 53.35; H, 5.66; N, 10.98. Found: C, 53.10; H, 5.70; N, 10.84. Electrochemistry (CH3CN): E1/2(MoVI/MoV) ) -0.65 V. 18O-Enriched LbMoVIO2(SPh) and [CoCp2][LbMoVO2(SPh)] were prepared by the methods described for the La analogues. [CoCp2][LcMoVO2(SPh)]. Method 1. A solution of CoCp2 (0.19 g, 1.0 mmol) in n-hexane (15 mL) was filtered into a Schlenk flask containing LcMoVIO2(SPh) (0.30 g, 0.56 mmol) and a magnetic stirring bar. Dropwise addition of dichloromethane (5 mL) to the stirred suspension produced the pale green product, which was filtered, washed three times with n-hexane, and dried under vacuum. Yield, 0.35 g (86%). Method 2. A solution of CoCp2 (0.09 g, 0.47 mmol) in toluene (5 mL) was added dropwise to a stirred solution of LcMoVIO2(SPh) (0.14 g, 0.26 mmol) in toluene (5 mL). The resultant green precipitate was filtered, washed with toluene, and dried under vacuum. Yield 0.17 g (90%). Anal. Calcd for C28H34BCoMoN9O2S: C, 46.30; H, 4.72; N, 17.35. Found: C, 46.03; H, 4.65; N, 17.11. Electronic spectrum (CH2Cl2): 330 (sh, ∼3000), 420 (sh, ∼1000), 780 (40), 1110 nm ( 30 M-1 cm-1). Electrochemistry (CH3CN): E1/2(MoVI/MoV) ) -0.53 V. [Na(thf)2][LaMoVO2(SPh)]‚xNaX (X ) Cl and/or OH). Tetrahydrofuran (20 mL) was added to a mixture of acenaphthalene (0.22 g, 1.4 mmol) and freshly cut sodium metal (0.17 g, 7.4 mmol) and the solution stirred at room temperature overnight. Unreacted Na metal was removed and the solution added to PPh4Cl (0.60 g, 1.6 mmol). The mixture was stirred at room temperature for 1 h before being filtered into a Schlenk flask containing LaMoVIO2(SPh) (0.60 g, 1.2 mmol). A yellow-green solid formed upon stirring. It was collected by filtration, washed with tetrahydrofuran (3 × 5 mL), and dried under vacuum. Yield 0.30-0.50 g (48-80%). Microanalysis. Sample 1. x ) 2.2 (X ) Cl). Calcd for C29H43BCl2.2MoN6Na3.2O4S: C, 42.00; H, 5.17; N, 10.13; S, 3.63. Found: C, 41.97; H, 5.22; N, 10.13; S, 3.86. Sample 2. x ) 1.6 (X ) Cl). Calcd for C29H43BCl1.6MoN6Na2.6O4S: C, 43.81; H, 5.45; N, 10.57; S, 4.03; Cl, 7.13. Found: C, 43.81; H, 5.44; N, 10.80; S, 3.93, Cl, 5.82. Infrared spectra (Nujol mull) were identical for samples 1 and 2: ν(B-H) 2512 m, ν(MoVO2) 853 m, 777 vs cm-1.

Xiao et al. “LaMoVO(OH)(SPh)”. A solution of CoCp2 (0.14 g, 0.74 mmol) in acetonitrile (10 mL) was added to a solution of LaMoVIO2(SPh) (0.20 g, 0.37 mmol) in dichloromethane (5 mL). After stirring at room temperature for 2 h, the volume was reduced to ca. 10 mL under reduced pressure. The green microcrystals were filtered, washed with acetonitrile, and dried under vacuum. Yield 0.08 g. An 18O-labeled sample was prepared similarily using LaMoVIO2(SPh) (55 atom % 18O). LaMoVO(OSiMe3)(SPh). A freshly deoxygenated solution of Me3SiCl (0.5 mL) in acetonitrile (15 mL) was added to [CoCp2][LaMoVO2(SPh)] (0.40 g, 0.55 mmol). The mixture turned dark green immediately. After stirring for 10 min, the solution was evaporated to dryness and the residue chromatographed in air on silica using 2:1 n-pentane: dichloromethane as eluent. The first dark green band was collected and a dark green solid isolated by addition of methanol. Yield, 0.31 g (92%). Anal. Calcd for C24H36BMoN6O2SSi: C, 47.45; H, 5.97; N, 13.83; S, 5.28. Found: C, 47.57; H, 5.69; N, 13.88; S, 5.20. Mass spectrum (m/z): 609. EPR (toluene): g, 1.942, A(95,97Mo), 44.0 × 10-4 cm-1. Infrared (KBr): ν(BsH) 2549 m, ν(SisC) 1249 m, ν(ModO) 953 s, ν(SisO) 920 s, br cm-1. Electrochemistry (CH3CN): MoV/ MoIV -0.89 (reversible) and MoVI/MoV +0.63 V (irreversible). 18OEnriched LaMoVO(OSiMe3)(SPh) was prepared similarly using [CoCp2][LaMoVO2(SPh)] (80 atom % 18O). Infrared (KBr): ν(BsH) 2548 m, ν(SisC) 1249 m, ν(Mod18O) 910 s, ν(Sis18O) 875 s, br cm-1. The minor product isolated from the second bright green band was identified as LaMoVOCl(SPh).37 LaMoVO(OSiMe3)(SPh) was prepared similarly from the salts [Na(thf)2][LaMoVO2(SPh)]‚xNaX (X ) Cl and/or OH) in ca. 64% yield. LbMoVO(OSiMe3)(SPh). The synthesis was similar to the La complex using [CoCp2][LbMoVO2(SPh)] as starting material. The product was purified by chromatography in air on silica using 2:1 n-pentane:dichloromethane as eluent. Yield 80%. Mass spectrum (m/ z): 651. EPR (toluene): g, 1.943, A(95,97Mo), 44.2 × 10-4 cm-1. Infrared (KBr): ν(BsH) 2488m, ν(SisC) 1247m, ν(ModO) 956s, ν(SisO) 915 s, br cm-1. Cyclic voltammetry (CH3CN): MoV/MoIV -0.79 (reversible) and MoVI/MoV +0.73 V (irreversible). LcMoVO(OSiMe3)(SPh). An analogous synthesis using [CoCp2][LcMoVO2(SPh)] as starting material produced the desired complex as major product, as demonstrated by thin layer chromatography of the initial product mixture. The complex is unstable in solution, and significant decomposition occurred during chromatographic workup on a silica column with acetonitrile as eluent. EPR (toluene): g, 1.944, A(95,97Mo), 44.0 × 10-4 cm-1. Infrared (KBr): ν(BsH) 2526m, ν(SisC) 1243m, ν(ModO) 956s, ν(SisO) 907s, br cm-1. Crystallography. Dark green crystals of [CoCp2][LcMoVO2(SPh)]‚toluene were obtained by dissolution in situ of the precipitate in synthetic method 2 by the addition of a minimum quantity of acetonitrile, followed by cooling of the solution at -20 °C for 3 days. A plate of approximate dimensions 0.40 × 0.38 × 0.06 mm was sealed in a Lindemann glass capillary tube under a dinitrogen atmosphere. Accurate values of the unit cell parameters and crystal orientation were obtained by a least-squares procedure from the angular settings of 25 carefully centered reflections. Intensity data were collected using an Enraf-Nonius CAD-4MachS single-crystal X-ray diffractometer using the ω:2θ scan method, with Mo KR radiation (graphite crystal monochromator) λ ) 0.71069 Å, at 294(1) K. Although protected from the atmosphere, the crystal decomposed significantly on exposure to the X-ray beam; three intensity control reflections, monitored every 9600 s of X-ray exposure time, showed a 35% deterioration in intensity. The data were corrected in accordance with this variation, as well as for Lorentz and polarization effects, but not for extinction. Analytical absorption corrections were applied, the maximum and minimum transmission factors being 0.951 and 0.756, respectively. Crystallographic data are summarized in Table 1. The structure was solved using a combination of Patterson map, direct methods, and difference syntheses.38 The positions of few (37) Cleland, W. E., Jr.; Barnhart, K. M.; Yamanouchi, K.; Collison, D.; Mabbs, F. E.; Ortega, R. B.; Enemark, J. H. Inorg. Chem. 1987, 26, 1017. (38) (a) Sheldrick, G. M. SHELXS86, Program for Crystal Structure Solution, Acta Cryst. 1990, A46, 467. (b) Sheldrick, G. M. SHELX76, Program for Crystal Structure Determination; Cambridge University: England, 1976.

Complexes Containing MoV Centers

J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2915

Table 1. Crystallographic Data for [CoCp2][LcMoVO2(SPh)]‚toluene formula color fw crystal system space group a, Å b, Å c, Å V, Å3 Z Fcalcd, g/cm3 radiation (λ), Å µ, cm-1 transmission factors, max min 2θ limits data collected no. of data collected no. of unique data Rint no. of data used in refinement final Ra final Rwb a

C35H42BCoMoN9O2S dark green 818.5 orthorhombic Pbca (No. 61) 17.695(3) 19.490(3) 21.925(4) 7561(2) 8 1.438 Mo KR (0.71069) 8.35 0.951 0.756 4.4 e 2θ e 50° h,k,l 8677 6617 0.019 2269 [I g 2.5σ(I)] 0.067 0.067

R ) ∑(|Fo| - |Fc|)/∑(Fo). Rw ) [∑w(|Fo| b

|Fc|)2/∑wFo2]1/2.

Table 2. Bond Distances and Angles for the Anion in [CoCp2][LcMoVO2(SPh)]‚toluene Distances (Å) Mo-O(1) 1.742(9) Mo-O(2) 1.742(9) Mo-S 2.442(4) Mo-N(11) 2.277(11) Mo-N(21) 2.410(12) Mo-N(31) 2.334(10) S-C(1) 1.811(15) O(1)-Mo-O(2) O(2)-Mo-S O(1)-Mo-N(21) O(2)-Mo-N(11) O(2)-Mo-N(31) S-Mo-N(21) N(11)-Mo-N(21) N(21)-Mo-N(31)

Angles (deg) 112.1(4) O(1)-Mo-S 95.5(4) O(1)-Mo-N(11) 84.3(5) O(1)-Mo-N(31) 90.4(5) O(2)-Mo-N(21) 87.5(5) S-Mo-N(11) 90.7(4) S-Mo-N(31) 80.9(6) N(11)-Mo-N(31) 75.0(5) Mo-S-C(1)

96.3(4) 90.1(5) 157.7(5) 161.6(5) 168.9(4) 91.9(4) 78.9(5) 110.5(6)

hydrogens could be found in the difference maps, and subsequently none were included in the model. Full-matrix least-squares refinement, with anisotropic temperature factors applied to each of the non-hydrogen atoms, and using a weighting scheme of type k[σ2(F) + gF2]-1, converged with R 0.067, Rw 0.067, k 2.699, and g 0.00047. The maximum peak heights in the final difference map were 0.68 e Å-3, close to the Mo atom, and 0.48 e Å-3 surrounding the toluene molecule, indicating slight disorder for this molecule. An analysis of variance showed no unusual features. The atomic scattering factors employed for non-metal atoms were those incorporated in the SHELX76 program.38b Coefficients for the scattering curves for atomic Co and Mo were obtained from ref 39; corrections were made for anomalous dispersion. Calculations were carried out on a Microvax II 630QB computer. Fractional atomic coordinates are available as supporting information. The structure and numbering scheme are represented using ORTEP.40 Selected bond distances and angles are given in Table 2.

Results Cobaltocenium Salts of [LMoVO2(SPh)]-. The [CoCp2]+/ CoCp2 couple occurs at E1/2 ) -0.94 V vs SCE in acetonitrile and so cobaltocene is thermodynamically capable of reducing LMoVIO2(SPh) [E1/2 ) -0.76 (La), -0.65 (Lb), -0.53 (Lc) V].23 Excess cobaltocene and precipitation of the product from (39) International Tables for X-Ray Crystallography; Kluwer: Dordrecht; 1993, (a) Vol. C, p 500, (b) Vol. C, p 219. (40) Johnson, C. K. ORTEPII. Report ORNL-5138; Oak Ridge National Laboratory: Oak Ridge, TN, 1976.

Figure 1. Cyclic voltammogram of [CoCp2][LaMoVO2(SPh)] in CH3CN. Conditions: glassy carbon electrode; scan rate ) 100 mV/s; 0.2 M Bun4NBF4; vs SCE.

acetonitrile ensured completion of the reaction for L ) La:

LaMoVIO2(SPh) + CoCp2 f [CoCp2][LaMoVO2(SPh)]

(1)

The equivalent salts for ligands Lb and Lc required toluene or alkanes as solvents as they were soluble in acetonitrile. All three salts were extremely sensitive to oxidation by dioxygen. Cyclic voltammograms of each salt at a glassy carbon electrode in acetonitrile revealed three reversible waves assignable to MoVI/MoV, [CoCp2]+/CoCp2, and CoCp2/[CoCp2]couples;22,23,41 the cyclic voltammogram of [CoCp2][LaMoVO2(SPh)] is shown in Figure 1. Addition of LMoVIO2(SPh) increases the peak current of the first couple while addition of CoCp2 increases the peak current of the latter two. The lower magnitude of the peak current of the first couple relative to the other two is consistent with lower diffusion coefficients for the molybdenum complexes relative to CoCp2. EPR data are summarized in Table 3. A freshly prepared solution of [CoCp2][LaMoVO2(SPh)] in carefully dried dichloromethane exhibited a very broad EPR signal at g ) 1.920 (Figure 2a). This signal was identical to that generated by reaction of LaMoVIO2(SPh) with excess Bun4NSH in THF and the initial signal observed upon electrochemical reduction of LaMoVIO2(SPh) in dry THF at room temperature;23 it is assigned to the anionic species [LaMoVO2(SPh)]-. However, at room temperature, the spectrum converted rapidly to a proton-coupled EPR signal (g ) 1.953, A(95,97Mo) ) 43.2 × 10-4 cm-1, A(1H) ) 13.1 × 10-4 cm-1) (Figure 2b), identical to that generated by electrochemical reduction of LaMoVIO2(SPh) in wet solvent.23 This signal is assigned to LaMoVO(OH)(SPh). When the solution was frozen, the highly anisotropic EPR signal characteristic of [LaMoVO2(SPh)]- 23 was observed (Figure 2c). The signal assigned to LaMoVO(OH)(SPh) was observed when the glass was allowed again to thaw to room temperature. These observations indicate the presence of an equilibrium between [LaMoVO2(SPh)]- and LaMoVO(OH)(SPh) in solution:

[LaMoVO2(SPh)]- + H2O h LaMoVO(OH)(SPh) + OH(2) The position of the equilibrium appears to be phase and/or temperature dependent. Similar behavior was observed in the case of [CoCp2][LbMoVO2(SPh)]. However, dissolution of [CoCp2][LcMoVO2(SPh)] in dichloromethane under the same conditions produced a clean EPR spectrum assignable to the (41) (a) Geiger, W. E., Jr. J. Am. Chem. Soc. 1974, 96, 2632. (b) Koelle, U. J. Organomet. Chem. 1978, 152, 225.

2916 J. Am. Chem. Soc., Vol. 118, No. 12, 1996

Xiao et al.

Table 3. EPR Data of [LMoVO2(SPh)]- in Dichloromethanea L

g1 (gzz)c

g2 (gyy)c

g3 (gxx)c

A1 (Azz)c

A2b (Ayy)c

A3 (Axx)c

g

A

W1/2 (G)

La Lb Lc L-N2S2d

1.991 1.994 1.994 1.987

1.931 1.938 1.936 1.916

1.843 1.842 1.844 1.811

27.9 29.0 27.0 27.1

28.4 ∼26 ∼25 31.2

67.9 ∼67 ∼68 68.8

1.920 1.920 1.923 1.904

41.4 40.6 40.0 42.4

20 20 19

a Anisotropic data obtained from frozen-glass samples at 77 K; isotropic data obtained from solutions at ambient temperatures. A (× 10-4 cm-1) values estimated directly from spectra. b Values estimated from 3A - A1 - A3. c Tentative assignments (see text). d From ref 26: complex implied is [(L-N2S2)MoO2]-.

Table 4. Infrared Assignments of νs(MoO2) and νas(MoO2) Modes (cm-1)a 16

O16O

complex a

VI

L Mo O2(SPh) LbMoVIO2(SPh) (L-NS2)MoVIO2b [LaMoVO2(SPh)]- c [LbMoVO

(SPh)]- c

2

[LcMoVO2(SPh)]- c

16

O18O

18

O18O

νs

νas

νs

νas

νs

νas

922 (923) 927 (926) 945 (945) 865(?) (865) 879 (879) 876

890

911

856

877

899

916

863

880

909

933

878

898

748

822

849 (848) 855 (856) 864 (867) 730 (731) ∼743 (741)

767 777

856

835

776

The νs values calculated using νs18/νs16 ) 0.950 and νas using eq A1 are given in parentheses. The observed O-Mo-O angle (θ) for LaMoVIO2(SPh) and [LcMoVO2(SPh)]- is used for LbMoVIO2(SPh) and [La,bMoVO2(SPh)]-, respectively. No labeling studies were possible for the Lc complex. b Reference 60. c Cobaltocenium salts. a

Figure 2. EPR spectra of [CoCp2][LaMoVO2(SPh)] in CH2Cl2: (a) freshly prepared solution at 293 K; (b) after standing for ca. 20 min at 293 K [spectra (a) and (b) have the same scale]; (c) solution (b) at 77 K.

anion [LcMoVO2(SPh)]-. This ion was stable under all conditions and did not undergo protonation. The relative stability of [LcMoVO2(SPh)]- permitted the growth of X-ray quality crystals of [CoCp2][LcMoVO2(SPh)]‚toluene. Infrared studies of 16O- and 18O-labeled [CoCp2][LMoVO2(SPh)] were undertaken to assist the assignment of the νs(MoO2) and νas(MoO2) vibration modes of the complex anions. Selected infrared spectral data are listed in Table 4. The theoretical treatments applied to the data are summarized in the Appendix. The spectrum of [CoCp2][LaMoVO2(SPh)] exhibited bands characteristic of the La ligand (ν(BH) 2524 m cm-1; ν(CN) 1542 s cm-1) and the cobaltocenium cation (3098, 1414, 1008, 858, and 460 cm-1).42 Examination of the 1100-450 cm-1 region of the spectrum (Figure 3a) revealed the absence of a strong absorption typical of the [MoVO]3+ unit between 1000 and 880 cm-1. However, a sharp, strong band was observed at 767 cm-1. When the sample was exposed to air, this band lost intensity and two absorptions typical of a cis-[MoVIO2]2+ center appeared at 922 and 890 cm-1 (Figures 3b and 3c). In 80 atom % 18O-labeled [CoCp2][LaMoVO2(SPh)], the 767 cm-1 band was virtually absent and two bands at 748 and 730 cm-1 were (42) The infrared bands for [CoCp2]+ were identified by comparison with the infrared spectra of [CoCp2]Cl and [CoCp2]Br.

Figure 3. Infrared spectra of [CoCp2][LaMoVO2(SPh)] (Nujol mull): (a) initial; (b) after standing in air for 6 min; (c) after standing in air for 30 min. Disappearance and in-growth of unobscured ν(MoO2) bands are indicated by arrows.

observed (Figure 4a); these are assigned to the νas(MoVO2) modes of the 16O18O and 18O18O isotopomers, respectively. Although obscured by other bands, there was evidence of a νs(MoVO2) band at ca. 865 cm-1 which shifted to 822 cm-1 in the 18O18O isotopomer (Figure 4a). The corresponding band for the 16O18O isotopomer was not observed presumably due to overlap with other absorptions.

Complexes Containing MoV Centers

J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2917

Figure 5. Structure of the anion in [CoCp2][LcMoVO2(SPh)]‚toluene. The numbering of the triazole ring containing N(21) parallels those shown for the other two rings.

with sodium acenaphthalenide in the presence of PPh4Cl in THF:

NaAc + LaMoVIO2(SPh) + PPh4Cl(s) f Figure 4. Infrared spectra of 16O- and 18O-substituted materials (Nujol mull): (a) [CoCp2][LaMoVO2(SPh)]; (b) [CoCp2][LbMoVO2(SPh)]. Unobscured ν(MoO2) bands only are shaded.

The above assigments were supported by equivalent experiments involving [CoCp2][LbMoVO2(SPh)]. Bands assignable to the νs(MoO2) absorptions of the 16O16O, 16O18O, and 18O18O isotopomers in 80 atom % 18O-labeled compound are observed at 879, 856, and 835 cm-1, respectively (Figure 4b), this time without interference from other bands. The νas(MoO2) modes of the compound are obscured somewhat but the 16O16O and 18O18O components of ν (MoO ) are estimated to occur close as 2 to 777 and 743 cm-1, respectively. Attempts to synthesize 18O forms of LcMoVIO2(SPh) were unsuccessful. The X-ray crystal structure of [CoCp2][LcMoVO2(SPh)]‚toluene has been communicated;29 details are summarized in Tables 1-2 and an ORTEP view of the anion is shown in Figure 5. We briefly reiterate the important features of this structure. The anion, in which molybdenum coordinates a facially tridentate triazolylborate ligand and mutually cis oxo and thiophenolate ligands, exhibits a distorted octahedral structure. The Mo atom is displaced from the midpoint of the equatorial plane defined by O(1), O(2), N(21), and N(31), toward the oxo ligands. The Mo-O(1) and Mo-O(2) bond distances are equal at 1.742 (9) Å, and the O(1)-Mo-O(2) angle is 112.1 (4)°. A noticeable trans influence is associated with the terminal oxo ligands. The Mo-S distance and Mo-S-C(1) angle are 2.442(4) Å and 110.5(6)°, respectively. The orientation of the phenyl group [Mo-S-C(1)-C(2) torsion angle -12.9(3)°] appears to be dictated primarily by the position of neighboring [CoCp2]+ counterions. A Complex Sodium Salt. The preparation of PPh4[LaMoVO2(SPh)] was attempted by reduction of LaMoVIO2(SPh)

PPh4[LaMoVO2(SPh)] + NaCl(s) + Ac (3) As is evident from the following properties, the green product was not PPh4[LaMoVO2(SPh)]: (i) Infrared bands characteristic of PPh4+ were absent. Infrared bands at 853 and 777 cm-1 were assigned to the νs(MoO2) and νas(MoO2) modes of the [LaMoVO2(SPh)]- anion; upon exposure to air, these bands disappeared and the equivalent absorptions of LaMoVIO2(SPh) grew in intensity. The compound was oxidized by air to LaMoVIO2(SPh). (ii) A single reversible couple at -0.76 V vs SCE was observed in the cyclic voltammogram in acetonitrile; it may be assigned to the LaMoVIO2(SPh)/[LaMoVO2(SPh)]couple.23 (iii) EPR spectra in dichloromethane were indistinguishable from those of [CoCp2][LaMoVO2(SPh)] (Figure 2). (iv) Reaction with Me3SiCl produced LaMoVO(OSiMe3)(SPh) in 64% isolated yield. Sodium was detected qualitatively,43 and microanalytical data were consistent with the presence of [Na(thf)2][LaMoO2(SPh)]‚xNaX (X ) Cl and/or OH; x ∼2). Although it was not incorporated into the product, PPh4+ appeared to be essential for the reproducible synthesis of this sodium salt. The susceptibility of the [LMoO2(SPh)]- anions to protonation suggested reactivity toward electrophilic reagents. The anions reacted cleanly with Me3SiCl, as shown in eq 4.

[LMoVO2(SPh)]- + Me3SiCl f LMoVO(OSiMe3)(SPh) + Cl- (4) When 18O-substituted anions were used, the reaction proceeded without loss of label. Bands characteristic of ν(MoVO) and ν(SiO) modes were observed. (43) Quantitative analysis of the content of sodium is not available due to the presence of boron and molybdenum in the sample.

2918 J. Am. Chem. Soc., Vol. 118, No. 12, 1996

Xiao et al.

Figure 7. Infrared spectra of the green solid (Nujol mull): (a) 100 atom % 16O; (b) ca. 55 atom % 18O. Unobscured bands which shift upon labeling are shaded in (a).

Figure 6. Infrared spectra of the green solid (Nujol mull): (a) initial; (b) after standing in air for 10 min; (c) after standing in air for 30 min. Peaks (*) and (‚) are assigned to [LaMoVO2(SPh)]- and LaMoVO(OH)(SPh), respectively. Decomposition of both compounds leads to the in-growth of peaks due to LaMoVIO2(SPh) (x).

LaMoVO(OH)(SPh).

A Product Containing Protonation of [La,bMoVO2(SPh)]- to form the conjugate acids La,bMoVO(OH)(SPh) has been demonstrated in dichloromethane (vide supra and ref 23). Attempts to isolate the acid forms from dichloromethane solution were unsuccessful. However, slow evaporation of dichloromethane from a solution of [CoCp2][LaMoVO2(SPh)] generated in a mixed acetonitrile:dichloromethane solvent system precipitated a green solid containing LaMoVO(OH)(SPh). This solid exhibited the electrochemical and EPR properties observed for [CoCp2][LaMoVO2(SPh)] (Figures 1 and 2). Its infrared spectrum (Figure 6) displayed the strong absorption at 767 cm-1 characteristic of [CoCp2][LaMoVO2(SPh)] and two strong bands at 915 and 535 cm-1, which were absent from the spectrum of [CoCp2][LaMoVO2(SPh)] (Figure 6a). Upon exposure to air, all three bands were replaced rapidly by the two bands characteristic of LaMoVIO2(SPh) (922 and 890 cm-1),22,23 which can be isolated from the oxidized product. The band at 915 cm-1 is typical of a ν(MoVdO) mode.37,44 The band at 535 cm-1 is plausibly a ν(MosOH) mode. Related LaMoVO(OMe)(SPh)44 and LMoO2(OMe)22,23 complexes each exhibit a strong band at ca. 540 cm-1, assignable to a ν(Mo-OMe) mode. Transition metal methoxo complexes generally exhibit strong absorptions in the range 300-600 cm-1.45 An 18O-labeled sample of the green solid was prepared from a L MoVIO2(SPh) (55 atom % 18O content). The infrared spectrum (Figure 7b) clearly revealed the appearance of a new band at 510 cm-1, assignable to the MoV-18OH isotopomer. Dilution of the label in the product may have resulted from oxygen exchange with the residual water in the solvents during (44) Xiao, Z.; Bruck, M. A.; Enemark, J. H.; Young, C. G.; Wedd, A. G. Manuscript in preparation. (45) Adams, R. W.; Martin, R. L.; Winter, G. Aust. J. Chem. 1967, 20, 773.

the reaction. The 915 cm-1 band appeared to be shifted to ca. 840 cm-1, but its clear identification was hampered by interfering bands. The bands assignable to the cis-[MoVO2]+ fragment in [CoCp2][LaMoVO2(SPh)] were also shifted as previously described. Discussion The few cis-[MoVIO2]2+ complexes which undergo a reversible, one-electron reduction appear to exhibit two important attributes: (i) minimal ligand conformational change upon reduction, thereby restricting substitution trans to the oxo groups, and (ii) the presence of a steric or electrostatic barrier to dimerization of the reduced species.20,22,23,46 The reversible one-electron chemistry seen for LMoVIO2(ER) (E ) O, S) can be attributed to the facial coordination enforced by L and the combined steric hindrance of L and ER.23,46 The one-electron reduced cis-[MoVO2]+ and cis-[MoVO(OH)]2+ species observed by EPR had never been isolated; in general, they are unstable unless generated under special conditions.23 In attempts to isolate cis-dioxo-Mo(V) and (hydroxo)oxoMo(V) complexes in substance, LMoVIO2(SPh) complexes were chosen as precursors, since their reduction potentials were more positive and their reduced forms more stable than those of other LMoVIO2(ER) species.22,23 A number of reductants were surveyed, but clean products were obtained with cobaltocene only. This reagent provides both the reducing equivalent and the countercation which rapidly precipitates [CoCp2][LMoVO2(SPh)] in these kinetically controlled syntheses. Rapid precipitation of the very unstable compounds is crucial. The dioxygenscavenging properties of cobaltocene ensure high yields of [CoCp2][LMoVO2(SPh)]. Unlike [La,bMoVO2(SPh)]-, which are protonated readily in solution, [LcMoVO2(SPh)]- is stable to protonation by adventitious water. This can be attributed to the stronger electron withdrawing properties of the triazolyl rings of Lc relative to the pyrazolyl rings of La,b and to the presence of the 4-N atoms which may compete with the oxo ligands for protons. These properties permit slow growth from the reaction (46) Xiao, Z.; Enemark, J. H.; Wedd, A. G.; Young C. G. Inorg. Chem. 1994, 33, 3438.

Complexes Containing MoV Centers mixture47 of crystals of [CoCp2][LcMoVO2(SPh)]‚toluene and the generation of clean EPR spectra. Each of the [CoCp2][LMo18O2(SPh)] salts is oxidized quantitatively to LMoVIO2(SPh) by dioxygen without loss of 18O label. Rapid precipitation of product is a feature of a number of other syntheses of unstable cis-dioxo-Ru(VI) and -Os(VI) complexes.7,12 Unfortunately, the ready precipitation of the [CoCp2][LMoVO2(SPh)] salts complicates the isolation of the cis-La,bMoVO(OH)(SPh) complexes in pure form. A green solid precipitated from a mixed solvent system appears to be a mixture of LaMoVO(OH)(SPh) and [CoCp2][LaMoVO2(SPh)]. This interpretation is consistent with the observations that the anion is protonated readily in solution and can be trapped in high yield by the electrophilic reagent Me3SiCl as the stable complex LaMoVO(OSiMe3)(SPh). These reactions also proceed without loss of 18O label. Infrared bands due to the νs and νas modes of the cis-dioxoMo(V) fragment have been assigned on the basis of detailed 18O-labeling studies. The lower energy of the bands relative to those of the Mo(VI) counterparts (Table 4) is consistent with a weakening of the ModO bonds upon reduction and single occupation of a π* MoO2 molecular orbital. This weakening is also reflected in increased Mo-O distances in the anion compared to its Mo(VI) analogue (vide infra). Significantly, the observation of bands assignable to νs(MoO2) and νas(MoO2) modes confirms the equivalence of the Mo-O bonds in the [LMoO2(SPh)]- complexes. A C1 [LMoO(O•)(SPh)]- structure having inequivalent Mo-O bonds, which might be envisioned to result from localization of the HOMO along one Mo-O direction, is inconsistent with the infrared data. Such a complex would be expected to exhibit a band typical of the [MoVO]3+ fragment, in the 900-1000 cm-1 region. The MO2 modes of the following compounds have been assigned as indicated: ReO2I(PPh3)2, 923 and 843 cm-1;3 [ReO2(bpy)(py)2]+, 904 and 845 cm-1 (a band at 767 cm-1 was initially4a assigned as a potential ReO2 mode);4 [RuO2(Me3tcn)(O2CCF3)]+, 856 and 842 cm-1;8 [RuO2(O2CMe)Cl2]-, 866 and 891 cm-1;7 [OsO2(bpy)2]2+, 883 and 863 cm-1.12 None of the above assignments were confirmed by labeling studies. At 870 and 770 cm-1, the νs(MoO2) and νas(MoO2) modes of the [LMoO2(SPh)]- complexes are the lowest in energy yet reported for a cis-dioxo species. The structures of [CoCp2][LcMoVO2(SPh)] and LcMoVIO2(SPh)23 were compared in a brief communication.29 At 1.742(9) Å, the Mo-O distances in [LcMoVO2(SPh)]- are slightly longer than the corresponding bonds of LcMoVIO2(SPh) (average 1.700(6) Å) and the median distance characteristic of oxo-Mo complexes (1.694 Å).48 Significantly, the two Mo-O bond distances are equivalent, and there is no unusual thermal behavior associated with the oxo ligands. As well, the O(1)Mo-O(2) angle of 112.1(4)° in [LcMoVO2(SPh)]- is increased considerably relative to that of 103.9(2)° in LcMoVIO2(SPh). This is associated with an increase in the N(11)-Mo-S angle to 168.9(4)° [cf. 152.7(1)° in LcMoVIO2(SPh)] and a conspicuous canting of the triazole planes about the Mo‚‚‚B vector in [LcMoVO2(SPh)]-. The structural features of LaMoVIO2(SPh)23 parallel those of LcMoO2(SPh) [Mo-O ) 1.696(4) Å, O-Mo-O (47) The instability of the complexes is evident in the slow (days) dissolution of freshly precipitated [CoCp2][LaMoO2(SPh)] in its mother liquor, with concomitant formation of crystals of [CoCp2]2[Co(SPh)4]. This compound, partially characterized by an X-ray diffraction study (Enemark, J. H.; Bruck, M., private communication) contains a previously reported anion: (a) Holah, D. G.; Coucouvanis, D. J. Am. Chem. Soc. 1975, 97, 6917. (b) Swenson, D.; Baenziger, N. C.; Coucouvanis, D. J. Am. Chem. Soc. 1978, 100, 1932. (48) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1.

J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2919

Figure 8. Influence of O-Mo-O angle on frontier orbital energies of LMoVIO2(SPh) complexes. The LUMO indicated would become the HOMO of a [LMoO2(SPh)]- species upon reduction. For the [LMoVO2(SPh)]- complexes, a 3a′(dyz) HOMO is consistent with the O-Mo-O angle of 112.1(4)° observed for [LcMoO2(SPh)]-.49 Only orbitals within the π-manifold of the MoO2 unit have been numbered, and only the π* orbitals are shown in this diagram. Predominant d-orbital contributors to the MOs are indicated in parentheses.

) 102.6(2)°], providing confirmation of shorter Mo-O distances and smaller O-Mo-O angles for the Mo(VI) Vs Mo(V) compounds. The modest lengthening and equivalence of the Mo-O bond distances in [LcMoVO2(SPh)]- are consistent with a principally metal-based 3-center π* HOMO. The increase in the O(1)-Mo-O(2) angle observed for the anion compared to the Mo(VI) analogues is suggestive of repulsion between the d1 electron and the MoO2 π-electrons. Assuming Cs local symmetry for the [LMoO2(SPh)]0/complexes, a π-orbital manifold, comprising three 3-center π-orbitals (2 × a′ + a′′), their π* partners (2 × a′ + a′′), and a nonbonding oxygen p-orbital combination (a′′), can be constructed from symmetry-adapted linear combinations of the four oxygen pπ and three molybdenum dπ orbitals. We define the x axis as being perpendicular to the mirror plane (yz) with the z axis bisecting the O-Mo-O angle, as shown in Figure 8. EHMO calculations suggest that the 3a′ and 3a′′ MoO2 π* orbitals of the LMoVIO2(SPh) complexes are close in energy and sensitive to changes in the O-Mo-O angle. For LMoVIO2(SPh), with an O-Mo-O angle of 103°,23 the dxy-based 3a′′ π* MO is predicted to be the LUMO (Figure 8a). However, if the O-Mo-O angle is increased to 112°, the dyz-based 3a′ orbital becomes the LUMO (Figure 8b). An increase in the O-Mo-O angle increases a′′ valence orbital overlap but decreases a′ valence orbital overlap. Consequently, the 3a′ π* MO is stabilized relative to the 3a′′ π* MO as the O-Mo-O angle increases. Importantly, the orbitals are very close in energy for all the reasonable O-Mo-O angles available to a cis-LMoO2 fragment. Given the limitations of EHMO calculations, either of the 3a′(dyz) or 3a′′(dxy) MOs could be the HOMO (magnetic orbital) in the [LMoVO2(SPh)]- complexes. Both would be compatible with the observed low-energy νs(MoO2) and νas(MoO2) modes and the nucleophilic behavior of the anions. However, the

2920 J. Am. Chem. Soc., Vol. 118, No. 12, 1996

Xiao et al.

increase in the O(1)-Mo-O(2) angle from 103.9(2)° in LcMoVIO2(SPh) to 112.1(4)° in [LcMoVO2(SPh)]- favors a 3a′(dyz) HOMO in the latter (cf. Figure 8b).49 Assuming this to be the case, the g and A(95,97Mo) tensor components can be calculated to first-order in spin-orbit coupling:50-52

2R22ξ ∆Exz

gzz ) 2.0023 -

gxx ) 2.0023 -

2R2δ2ξ 6R2γ2ξ ∆Ex2-y2 ∆Ez2 2R2β2ξ ∆Exy

gyy ) 2.0023 -

[

Azz ) P -R2κ + (gzz - 2.0023) +

[

{

2R2 3ξ R2β2 R2γ2 R2δ2 + + 7 7 ∆Exy ∆Ez2 ∆Ex2-y2

Axx ) P -R2κ + (gxx - 2.0023) -

[

{

4R2 3ξ R2β2 R22 + 7 7 ∆Exy ∆Exz

Ayy ) P -R2κ + (gyy - 2.0023) +

{

2R2 3ξ R2γ2 R2δ2 R22 + + 7 7 ∆Ez2 ∆Ex2-y2 ∆Exz

}] }] }]

where ∆Ei ) Ei - Eyz, ξ is the spin-orbit coupling constant for MoV in the complexes, P ) gegNβeβN〈r-3〉4d, and κ is the isotropic hyperfine constant in the complexes. As a consequence of the σ* nature of dxz, which is directed at two oxygen and two nitrogen atoms, ∆Exz will be large and gzz is predicted to be close to the free-electron g value. On the other hand gxx and gyy will be significantly lowered by spin-orbit coupling and the energetic accessibility of the dxy and dz2 orbitals. Thus, we conclude that gzz > gxx, gyy. Accordingly, gzz may be confidently assigned to g1 in Table 3. This assignment is (49) The results of the EHMO calculations and the choice of a dyz-based magnetic orbital are supported by near-IR/IR studies of [LcMoVO2(SPh)](which is more stable than the other species and resistant to protonation in solution). Transitions at ca. 2800 and 8500 cm-1 are predicted on the basis of the EHMO calculations summarized in Figure 8b. Near-IR/IR spectroelectrochemistry of [LcMoVIO2(SPh)]0/- (50 mM, 0.15 M Bun4NBF4/CD2Cl2) reveals spectral changes in the 3000 to 3500 cm-1 region, which we tentatively ascribe to a band in that region of the spectrum of [LcMoVO2(SPh)]-. A band at 9000 cm-1 (1110 nm) is definitely present in the near-IR spectrum of [CoCp2][LcMoVO2(SPh)] (Experimental Section). The generation of the anion in the spectroelectrochemical experiment was confirmed by the appearance of the near-IR band at 9000 cm-1. (50) McGarvey, B. R. In Transition Metal Chemistry; Carlin, R. L., Ed.; Dekker: New York, 1966; Vol. 3, pp 89-201. (51) Mabbs, F. E.; Collison, D. Electron Paramagnetic Resonance of d Transition Metal Compounds; Elsevier: Amsterdam, 1992. (52) In the derivation of these equations, the effects of covalent bonding and symmetry allowed d-orbital mixing are ignored, viz., orbitals of the following form are employed (in increasing energy): HOMO ) φ1 ) Rdyz < φ2 ) βdxy < φ3 ) γdz2 < φ4 ) δdx2-y2 < φ5 ) dxz. The possible d orbital mixings in the antibonding orbitals are the following: HOMO ) φ1 ) R[a1dyz + b1dz2 + c1dx2-y2], a1 > b1, c1; φ2 ) β[e2dxy + f2dxz], e2 > f2; φ3 ) γ[a3dyz + b3dz2 + c3dx2-y2], b3 > a3, c3; φ4 ) δ[a4dyz + b4dz2 + c4dx2-y2], c4 > a4, b4; φ5 ) [e5dxy + f5dxz], f5 > e5. Complete expressions for gii and Aii may be developed using the treatments given in refs 50 and 51 (Chapter 9), but without additional information (e.g., orbital coefficients, assigned spectra, etc.) they provide little further insight into possible assignments.

consistent with the observation of 35,37Cl superhyperfine coupling in the g1 component of [LcMoVO2Cl]-,23 as overlap of the HOMO and the pz orbital of Cl (sited on the y axis) provides a mechanism for coupling in this direction. Further, from the equations for Aii above, it is predicted that |Axx| > |Azz|, |Ayy|. This permits the assignment of gxx as g3 in Table 3, as this component of g is associated with the largest component of A. As well, superhyperfine coupling to 17O is predicted to be strongest in the y direction when the HOMO is the dyz orbital, i.e., the largest 17O superhyperfine component should be associated with g2 ) gyy. For [(L-N2S2)MoVO2]-, the only cisdioxo-Mo(V) complex for which 17O coupling data are available, the largest 17O superhyperfine coupling is associated with g2.27 Previously, an assignment of the EPR spectrum of [(L-N2S2)MoVO2]- was made assuming a dxy-based magnetic orbital.26,27 The possibility of a dyz-based ground state for [(L-N2S2)MoVO2]- should now be considered,53 given the bonding implications of the structure of [LcMoVO2(SPh)]- reported herein. Recent ab initio and INDO MO calculations2h on [(LN2S2)MoVO2]- and [(L-N2O2)MoVO2]- support a close structural and electronic relationship between [(L-N2S2)MoVO2]- and the [LMoVO2(SPh)]- complexes reported herein.54 The isolation and full characterization of the [CoCp2][LMoVO2(SPh)] complexes provides strong confirmation of the essential nature of previously postulated cis-dioxo-Mo(V) complexes.20,21,24-27 Nucleophilic attack by [LMoVO2(SPh)]- leads (variously) to LMoVO(OR)(SPh) complexes (R ) H, SiMe3, Me). We have presented evidence to support the first isolation of a cis(hydroxo)oxo-Mo(V) complex, LaMoVO(OH)(SPh), albeit in a form contaminated by its conjugate base. The cis structure is proposed on the basis of the facial coordination of La and the crystal structures of the related LaMoVO(OSiMe3)(SCH2Ph) and LaMoVO(OSiMe3)(NCS) complexes.55 It appears that [CoCp2][LaMoVO2(SPh)] and LaMoVO(OH)(SPh) coprecipitate as a consequence of the equilibrium in eq 2. The EPR spectra of LaMoVO(OH)(SPh) are related to those of previously in situ generated [MoO(OH)]2+ complexes.23 Protonation and other nucleophilic reactions of the cis-[MoO2]+ moiety may be promoted by the spectator oxo effect described by Rappe and Goddard.2d,e Conclusion Careful control of the steric and electronic properties of ligands L and X in the [LMoO2X]0/- species22,23 has permitted the isolation of a number of cis-dioxo-Mo(V) complexes for the first time. Fine-tuning of the basicity of L to favor the [MoVO2]+ fragment relative to its conjugate acid, coupled with a careful choice of X to disfavor condensation, was the key to the isolation of these highly oxygen-sensitive species. One(53) In the coordinate system adopted, the following predictions are made for an [(L-N2S2)MoVO2]- complex having a dxy-based magnetic orbital: gxx > gyy > gzz and |Azz| > |Axx|,|Ayy|.26 Assuming [(L-N2S2)MoVO2]- to have a dyz-based magnetic orbital leads to the same predictions made here for the [LMoVO2X]- complexes. Definitive assignments in both systems require single crystal data for the [MoVO2]+ species doped into a suitable diamagnetic lattice. (54) Recent MO calculations on related [(L-N2S2)MoO2]- and [(L-N2O2)MoO2]- complexes suggest that the electronic structures of dioxo-Mo(V) complexes are also critically dependent on the nature of the coligands.2h,36 For the N2S2-donor complex a metal(4d/5p)-based HOMO (magnetic orbital), a Mo-O distance of 1.69 Å, and a O-Mo-O angle of 109° are predicted. The Mo-O distance is essentially identical to that calculated for the Mo(VI) analogue (L-N2S2)MoO2 but the O-Mo-O angle is considerably larger in the Mo(V) complex. The N2O2-donor complex [(LN2O2)MoO2]- is predicted to have a HOMO with significant oxygen p-orbital character, two distinctly different Mo-O distances (1.68 and 1.87 Å) and an O-Mo-O angle of 103°. The structural parameters in this case are distinctly different from those of the analogous Mo(VI) complex. (55) Bruck, M.; Enemark, J. H. Unpublished results.

Complexes Containing MoV Centers

J. Am. Chem. Soc., Vol. 118, No. 12, 1996 2921

electron reduction of LcMoVIO2(SPh) results in a lengthening of the Mo-O bond lengths and an opening of the O-Mo-O bond angle, but the two oxo ligands remain equivalent. Correspondingly, the νs(MoO2) and νas(MoO2) modes of the [LMoVO2X]- complexes are significantly lowered in energy. One of the oxo ligands in the anions may be protonated or silylated due to their increased basicity. The acid LaMoVO(OH)(SPh) was also isolated for the first time as a coprecipitate with [CoCp2][LaMoVO2(SPh)]. EPR studies of various pterincontaining molybdoenzymes indicate that (hydroxo)oxo-Mo(V) centers are generated upon enzyme reduction.27,32,33 However, an EPR signal indicative of a dioxo-Mo(V) center has not been reported for any enzyme. Our results demonstrate that the basicity of the [MoVO2]+ center may be electronically finetuned by the coligands. The molybdenum centers in enzymes, it seems, possess coligands which impart high basicity to any nascent [MoVO2]+ center and promote its rapid protonation. This is, of course, a key to the proposed coupled electron-proton transfer steps which appear to regenerate enzyme active sites following oxygen atom transfer between water and substrate.15,16 Acknowledgment. We thank Prof. A. Nakamura for providing access to the CAChe workstation on which EHMO calculations were performed, the Japan Society for the Promotion of Science for a fellowship enabling C.G.Y. to visit Japan, Dr. Stephen P. Best for spectroelectrochemical experiments, and Prof. John H. Enemark for helpful discussions. We gratefully acknowledge the financial support of the Australian Research Council. Appendix cis-[MoVIO2]2+ complexes typically exhibit νs(MoO2) and νas(MoO2) modes separated by about 30 cm-1 in the range 870950 cm-1 with νs at higher energy.56,57 The frequencies of modes νas and νas′ of nonlinear harmonic oscillators YXY and isotopomer Y′XY′ are approximated by eq A1:

( )

νas′ 2 my(mx + 2my′ sin2 θ) ) νas my′(mx + 2my sin2 θ)

(A1)

where mx, my, and my′ are atomic masses and θ is half the YXY angle.58,59 Assuming that cis-[MoO2]2+ fragments are inde(56) Griffith, W. P.; Wickins, T. D. J. Chem. Soc. A 1968, 400. (57) Griffith, W. P. J. Chem. Soc. A 1969, 211. (58) Herzberg, G. Infrared and Raman Spectra of Polyatomic Molecules; D. Van Nostrand: Princeton, NJ, 1945; p 228 and 231. (59) Pinchas, S.; Laulicht, I. Infrared Spectra of Labelled Compounds; Academic: New York, 1971; pp 27-28.

pendent harmonic oscillators in their complexes, then νas′ for the cis-[Mo18O2]2+ center can be estimated from assigned νas values in cis-[Mo16O2]2+ complexes. Results calculated in the present work for La,bMoVIO2(SPh)22,23 and (L-NS2)MoVIO260 are consistent with the assignments (Table 4). Assuming the O-Mo-O bond angle of 112.4° observed in [CoCp2][LcMoVO2(SPh)] applies to the La and Lb salts, estimation of νas(Mo18O2) in [LaMoVO2(SPh)]- using eq A1 is in excellent agreement with the assignments (Table 4). An equivalent calculation for [LbMoVO2(SPh)]- predicts νas(Mo18O2) to be close to the approximate value estimated after allowing for overlap with other absorptions (Table 4; Figure 4b). Similar assumptions to those used to derive eq A1 lead to eq A2:

( )

νs′νδ′ 2 my2(mx + 2my′) ) νsνδ my′2(mx + 2my)

(A2)

where νs and νδ are YXY symmetric stretching and deformation modes, respectively.58,59 It follows that, for any two symmetrically labeled YXY fragments A and B, eq A3 holds:

( )( ) ( )( ) νs′ νs

A

νδ′ νδ

)

A

νs′ νs

B

νδ′ νδ

(A3)

B

Assuming (νδ′/νδ)A ) (νδ′/νδ)B, then (νs′/νs)A ) (νs′/νs)B. This appears to be true for cis-[MoVIO2]2+ centers: for the nine cis[MoVIO2]2+ complexes reported in Table 4 and references 61 and 62, (νs18/νs16) ) 0.950 ( 0.001. Absorptions assignable to νs(Mo18O2) are clearly seen in the spectra of [CoCp2][La,bMoVO2(SPh)] (Table 4, Figure 4) at 822 and 835 cm-1, respectively. Application of the above ratio, derived for MoVI, to these MoV species predicts νs(Mo16O2) values close to those assigned tentatively (Table 4). Supporting Information Available: Tables of crystal data, final atomic coordinates, anisotropic temperature factors, and bond lengths and angles (10 pages) for [CoCp2][LcMoO2(SPh)]‚toluene. This material is contained in many libraries on microfiche, immediately follows this article in the microfilm version of the journal, can be ordered from the ACS, and can be downloaded from the Internet; see any current masthead page for ordering information and Internet access instructions. JA952525A (60) Craig, J. A.; Holm, R. H. J. Am. Chem. Soc. 1989, 111, 2111. (61) (a) Newton, W. E.; McDonald, J. W. Proceedings of the Second International Conference on the Chemistry and Uses of Molybdenum; Mitchell, P. C. H., Seaman, A., Eds.; Climax Molybdenum Company: Michigan, 1976; pp 25-30. (b) Newton, W. E.; McDonald, J. W. J. LessCommon Met. 1977, 54, 51. (62) Corbin, J. L.; Miller, K. F.; Pariyadath, N.; Wherland, S.; Bruce, A. F.; Stiefel, E. I. Inorg. Chim. Acta 1984, 90, 41.